Typically, computer systems store data on a magnetic medium such as a hard disk drive. The hard disk drive is an electromechanical component of the computer system that operates by storing polarities on magnetic material which can be rewritten quickly and as often as desired. A typical hard drive includes at least two moving parts that respond to control signals generated by a processor of the computer system. Conventionally, a hard disk drive includes a disk that is formed from an aluminum substrate that is rotatably mounted to a base. A magnetic material is deposited on a surface of the substrate. A rotatable actuator arm moves a ceramic transducer over the surface of the disk to read data from and write data to the hard disk. These mechanical parts are more delicate and less reliable than the other, solid state, components of the computer system. However, magnetic hard disk systems have dominated storage media for computers and related systems due to the low cost per bit of data and high density storage capacity of available magnetic hard disk systems when compared to conventional solid state alternatives.
Solid state memory devices store data in storage locations, referred to as "cells." Conventional designs only allow a single bit of data to be stored at a given time in each cell. Typically, a cell includes an access transistor and a storage element such as a capacitor or a floating gate that stores data based on the electric charge on the storage element. The electric charge in conventional applications represents either a binary "1" or "0" and thus conventional designs require a single transistor for each bit of data. The storage density for solid state memories is limited by the ability of designers to pack transistors on a semiconductor substrate. Although transistors can be packed more tightly together with each succeeding generation of design technology, this density does not compare well with the storage density of a magnetic medium.
Recently, designers have attempted to increase the storage density of flash memory cells by creating a memory cell that is capable of storing more than one data bit--so called "multi-state" flash memory cells. In a conventional flash memory, charge is stored on a floating gate of a field-effect transistor in response to a signal applied to a control gate. The charge on the floating gate represents either a binary "1" or "0" based on the effect the charge has on the current through the transistor. Initially, the floating gate is not charged, which represents a binary "1." When a binary "0" is stored, electrons are forced to the floating gate by a sufficient voltage on the control gate to induce hot electron injection which reduces the drain current of the transistor. Thus, by sensing the drain current of the transistor, the value of the data bit stored by the flash memory cell can be determined.
To increase the number of states that can be stored, designers have attempted to use adjustments to the threshold voltage of the transistor. Unfortunately, this technique has only been shown to work with, at most, storing two to four bits of data in a single cell due to variations in threshold voltage of each transistor in the array of memory cells. Otherwise, complex programming techniques to adjust the threshold voltage of the transistors during each read and write operation must be used.
In S. Tiwari et al, Volatile and Non-Volatile Memories in Silicon with Nano-Crystal Storage, IEEE Int. Electron Device Meeting, pp. 521-524 (1995) (hereinafter "Tiwari"), researchers from IBM describe an alternative structure for a flash memory device. The proposed memory device includes a floating gate that is formed within 15 to 25 .ANG. of a surface of the semiconductor substrate. The floating gate is formed from so called "nano-crystals" that trap electrons by a tunneling effect This structure is not practical due to difficulties in fabricating a gate oxide with a thickness on the order of 4 to 5 atoms. It also does not address the problem of storing multiple data bits in a single memory cell.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a solid state memory device that is capable of storing multiple bits in each memory cell.